1. Field of the Invention
The present invention relates to a method for hydroprocessing a heavy hydrocarbon oil, in particular a method in which a combination of two catalysts is used to obtain advantageous effects in the hydroprocessing of heavy hydrocarbon oils. The present invention also relates to a combination of catalysts suitable for use in such a process.
2. Prior Art
Japanese patent No. 765055 discloses a process for hydroprocessing a heavy hydrocarbon oil containing sulfur impurities and metallic impurities at least in two stages. In the first stage a catalyst containing 0.1 to 5 wt. % of metal oxide was used to effect hydrodemetallization and hydrocracking. In the second stage, a catalyst containing 7 to 30 wt. % of a metal oxide was used to effect hydrodesulfurization. However, due to, among other features, the high metals content of the second catalyst, asphaltene conversion in this process is not adequate.
Japanese patent No. 8325580 A discloses a catalytic hydroconversion method for a heavy feedstock. In the first stage a catalyst with a carrier material selected from alumina, silica, and a combination thereof which comprises 2 to 25 wt. % of oxides of active metals selected from cadmium, chromium, cobalt, iron, molybdenum, nickel, tin, tungsten or a combination thereof, is applied at a reaction temperature of 438 to 468° C., a hydrogen partial pressure of 105 to 245 kg/cm2, and a volume hourly space velocity of 0.3 to 1.0 h-1. In the second stage a similar catalyst is used at a reaction temperature of 371 to 427° C., a hydrogen partial pressure of 105 to 245 kg/cm2, and a space velocity of 0.1 to 0.8 h-1. The first stage is operated at a higher temperature and a lower catalytic activity than the second stage to improve hydroconversion.
However, it appears that the high temperature applied in the first stage leads to thermal condensation of asphaltene on the one hand and, on the other, the petroleum resin fractions produced with the thermal cracking of oil are unstable. This can lead to undesired cohesion and precipitation of asphaltene and coke formation in the second stage.
Japanese patent publication 653875 describes a multistage catalytic method for the high conversion of a heavy hydrocarbon liquid feedstock. In the first stage, a fixed bed or ebullated bed reactor is operated at a reaction temperature of 415 to 455° C., a hydrogen partial pressure of 70 to 211 kg/cm2, and a space velocity of 0.2 to 2.0 h-1. In the second stage, an ebullated bed reactor is operated under similar conditions. The catalyst used in the above method comprises a carrier material selected from alumina, silica, and a combination thereof, and the oxide of an active metal selected from cadmium, chromium, cobalt, iron, molybdenum, nickel, tin, tungsten, and mixtures thereof. In this reference, vacuum bottoms are recirculated to achieve a high cracking rate, but the problem of asphaltene cohesion accompanying the high cracking rate operation is not solved.
In summary, the processes of the prior art are inadequate in inhibiting sediment formation in the hydroprocessing of heavy oil to effect hydrodesulfurization and conversions.